Formation and structure of inclusion complexes involving α-cyclodextrin and alkyl(aqua)cobaloxime

Abstract

A series of inclusion complexes involving α-cyclodextrin (α-cd) and alkyl(aqua)cobaloximes has been synthesized for the first time and characterized by elemental analysis, one and two-dimensional ¹H NMR spectroscopy. Their formation constants K_a were determined by quantitative ¹H NMR methods. The crystal structures of α-cd–[Co(Hdmg)₂Prⁿ(H₂O)](H₂dmg = dimethylglyoxime) and its guest molecule [Co(Hdmg)₂Prⁿ(H₂O)] were determined. According to both NMR and crystal structure data, 1 : 1 inclusion complexes of α-cd and alkylcobaloximes are formed, in which the alkyl groups of the guests are included into the cavity of α-cd. After formation of the inclusion complexes, variations in the bond lengths and angles around the cobalt atom, and conformation changes of the n-propyl(aqua)cobaloxime were observed. It is suggested that these changes in the guest molecule are probably due to the hydrophobic interaction between the alkyl groups and the α-cd cavities, and steric interaction between α-cd and Co(Hdmg)₂. It has also been established that the stability of the inclusion complexes in aqueous solution is related to the length and the size of the alkyl groups, the K_a values varying in the order n-C₅H₁₁ Buⁿ > Prⁿ > Buⁱ.